Category: Class 12 Physics Objective Questions

Physics Assessment Questions for Class 12 on “The Line Spectra of the Hydrogen Atom”.

1. What causes spectral lines?
a) The transition of electrons between two energy levels
b) The transition of electrons between two wavelength ranges
c) Magnetic and electric field exiting in an atom
d) The transition of electrons from electric to magnetic field
Answer: a
Clarification: The observed spectral lines are caused by the transition of electrons between two energy levels in an atom. The emission spectrum of the hydrogen atom is divided into many spectral series, with wavelengths that are given by Rydberg’s formula.

2. How many spectral lines are there in the hydrogen spectrum?
a) Infinity
b) Zero
c) Multiple
d) One
Answer: c
Clarification: Even though hydrogen has only one electron in its outermost shell, it has multiple spectral lines. This is because hydrogen has many energy levels. When the electron excites from a lower level of energy into a higher level, then it releases a photon, and this photon is the one that appears as spectral lines.

3. Which theory explained that electrons revolved in circular orbits?
a) Einstein theory
b) Bohr theory
c) Rydberg theory
d) de – Broglie theory
Answer: b
Clarification: Niels Bohr explained the line spectrum of the hydrogen atom with the assumption that electrons revolved around an atom in circular orbits and that the orbit closer to the nucleus represented the ground state and the farther orbits represented the higher levels of energy.

4. Bohr’s model only works for hydrogen or helium.
a) True
b) False
Answer: a
Clarification: Yes, this is a true statement. Bohr’s model works only for elements like hydrogen because the model only considers the interactions between one electron and the nucleus. If there are more electrons, then they will repel the one electron, and this, in turn, will change its energy level.

5. When is hydrogen stable?
a) When the electron jumps to higher energy levels
b) When an electric field is introduced
c) When a magnetic field is introduced
d) When the electron is at its ground state
Answer: d
Clarification: The hydrogen atom is stable when the electron rests at the ground level of energy, i.e. when the principal quantum number, n = 1. In other words, when the electron is revolving in the first orbit around the nucleus, the hydrogen atom is stable.

6. Which of the following is found in the UV region of the spectrum?
a) Pfund series
b) Brackett series
c) Lyman series
d) Paschen series
Answer: c
Clarification: The transitions that end at the ground level, where the principal quantum number is one, are called the Lyman series. Here, the energies that are released are so large, and therefore, these spectral lines appear in the ultraviolet region of the spectrum.

7. How many spectral lines does hydrogen have?
a) Four
b) Three
c) Two
d) One
Answer: a
Clarification: Niels Bohr calculated the energies that a hydrogen atom would have in each of its energy levels, based on the wavelength of the spectral lines. Then he found out that there are four spectral lines for hydrogen, namely, Lyman, Balmer, Paschen, and Brackett series. The Lyman series lies in the UV region, whereas the Balmer series lies in the visible region, and the last two lie in the infrared region.

Physics Assessment Questions for Class 12,

Physics Multiple Choice Questions on “Dielectrics and Polarisation”.

1. In which type of molecule positive and negative charges coincide with each other?
a) Polar
b) Unipolar
c) Non-polar
d) Bipolar
Answer: c
Clarification: A molecule in which the centre of mass of positive and negative charges collide with each other is called a non-polar molecule. They normally have zero dipole moment. They have symmetrical shapes.

2. What is the ratio of the polarization to ε_o times the electric field called?
a) Polarisation density
b) Electric susceptibility
c) Dielectric strength
d) Dielectric susceptibility
Answer: b
Clarification: The ratio of the polarization to ε_o times the electric field is called the electric susceptibility of the dielectric. It describes the electrical behaviour of a dielectric.

3. Which of the following is an example of a molecule whose centre of mass of positive and negative charges coincide each other?
a) CO₂
b) CO
c) CH₃OH
d) NH₃
Answer: a
Clarification: CO₂ is a molecule in which the centre of mass of positive and negative charges collide with each other and is called a non-polar molecule. They normally have zero dipole moment. They have symmetrical shapes.

4. What is the induced dipole moment developed per unit volume of a dielectric when placed in an external electric field called?
a) Relative permittivity
b) Polarisation susceptibility
c) Electric susceptibility
d) Polarisation density
Answer: d
Clarification: The induced dipole moment developed per unit volume of a dielectric when placed in an external electric field is called polarization density. It may be defined as the charge induced per unit surface area.

5. In which type of molecule positive and negative charges does not coincide with each other?
a) Isentropic
b) Equipotential
c) Polar
d) Non-polar
Answer: c
Clarification: A molecule in which the centre of mass of positive and negative charges does not collide with each other is called a polar molecule. They have a permanent dipole moment. They have unsymmetrical shapes.

6. The molecules of a polar dielectric have no dipole moment. State true or false.
a) True
b) False
Answer: b
Clarification: The molecules of a polar dielectric have permanent dipole moments. In the absence of an external electric field, the dipole moments of different molecules are randomly oriented due to thermal agitation in the material.

7. ‘X’ is a substance which does not allow the flow of charges through it but permits them to exert electrostatic forces on one another through it. Identify X.
a) Polar molecule
b) Dielectric
c) Non-polar molecule
d) Equipotential
Answer: b
Clarification: A dielectric is a substance which does not allow the flow of charges through it but permits them to exert electrostatic forces on one another through it. A dielectric is essentially an insulator which can be polarized through small localized displacements of its charges.

8. Which of the following is an example of a molecule whose centre of mass of positive and negative charges does not coincide each other?
a) NH₃
b) H₂
c) CH₄
d) CO₂
Answer: a
Clarification: NH₃ is a molecule in which the centre of mass of positive and negative charges does not collide with each other and is called a polar molecule. They have a permanent dipole moment. They have unsymmetrical shapes.

Physics Exam Questions for Schools on “Current Electricity – Cells, Emf and Internal Resistance”.

1. Which of the following is the correct statement regarding electrochemical cell?
a) It converts chemical energy to electrical energy
b) It converts electric energy to chemical energy
c) It converts chemical energy to thermal energy
d) It does not maintain the flow of charge in a circuit
Answer: a
Clarification: An electrochemical cell is a device which converts chemical energy to electric energy, and maintains the flow of charge in a circuit. There are 2 types of electrochemical cells – Galvanic cell and Electrolytic cell.

2. Pick out the dimensional formula of emf from the following.
a) [M¹L²T³A¹]
b) [ML³T³A¹]
c) [M²L²T¹A^-1]
d) [ML²T^-3A^-1]
Answer: d
Clarification: Electromotive force (emf) is defined as the potential difference between the two terminals of a cell in an open circuit. The SI unit of emf is (frac {joule}{coulomb}) or volt. So, its dimensional formula is [ML²T^-3A^-1].

3. Which is the factor that internal resistance does not depend on?
a) Distance between the electrodes
b) Temperature of the electrolyte
c) Nature of electrode and electrolyte
d) Area of the electrode, immersed in the electrolyte
Answer: b
Clarification: Internal resistance is defined as the resistance offered by the electrolyte and electrodes of a cell when the current flows through it. Internal resistance depends on distance between the electrodes, the nature of electrodes and electrolyte, and area of the electrode immersed in the electrolyte. So, that leaves temperature of the electrolyte out, which is the answer.

4. The emf of a cell depends upon concentration of the electrolyte.
a) True
b) False
Answer: a
Clarification: The emf of a cell depends upon the nature of electrodes, nature and concentration of electrolyte used in the cell and its temperature as well. Emf of a cell is inversely proportional to the concentration of the electrolyte.

5. Identify the correct statement from the following about discharging of a cell.
a) The direction of current in the cell is from positive to negative terminal
b) Terminal potential difference is greater than emf of the cell
c) Terminal potential difference is lesser than emf of the cell
d) The current increases and decreases frequently
Answer: c
Clarification: During discharging of a cell terminal potential difference, the terminal potential difference is lesser than the emf of the cell. The direction of current inside the cell is from negative terminal to positive terminal.

6. A current of 3 A passes through an electric circuit for 5 minutes and does a work of 900J. What is the emf of the source?
a) 3V
b) 1V
c) 5V
d) 10V
Answer: b
Clarification: Current = 3 A; Time taken = 5 minutes = 300 seconds
Work done = 900 J; Power = (frac {Work , done}{Time , taken} = frac {900}{300}) = 3 W
Power = Voltage (emf) x Current → Emf = (frac {Power}{Current} = frac {3}{3}) = 1V
Therefore, the emf of the source is 1 volt.

7. The emf of a battery is 86V and internal resistance 1 ohms in the figure shown below. Calculate the current drawn from the battery.

a) 2 A
b) 3 A
c) 5 A
d) 6 A
Answer: d
Clarification: The resistances 8 ohms, 10 ohms, and 2 ohms are in series, so the equivalent resistance is: R_S = 8 + 10 + 2 = 20 ohms; R_S and 4 ohms are in parallel connection to each other, so,
(frac {1}{R_P} = frac {1}{20} + frac {1}{4} = frac {3}{10}) → R_P = (frac {10}{3})
Total resistance (R) = 3 + (frac {10}{3}) + 7 + 1 (internal resistance) = (frac {43}{3})
According to Ohm’s Law → V = IR → I = (frac {V}{R} = frac {86}{frac {34}{3}})
= 3 × 2
= 6 A
Therefore, the current drawn from the battery is 6 amperes.

8. A cell has an emf of 6V, internal resistance of 1 ohms and a current of 0.5 A passing through it. This cell is connected to a resistor. Find out the resistance of the resistor.
a) 10 ohms
b) 11 ohms
c) 12 ohms
d) 13 ohms
Answer: b
Clarification: Emf (e) = 6V; Internal resistance (r) = 1 ohms; Current (I) = 0.5A
Required equation: I = (frac {e}{R}) + r
0.5 = (frac {6}{R}) + 1
R = (frac {(6 – 0.5)}{0.5})
R = 11 ohms
Therefore, the resistance of the resistor is 11 ohms.

9. A group of N cells such its emf E_N = 1.5r_N is shown in the diagram below. What is the current I in the circuit?

a) 51 A
b) 0.015 A
c) 1.5 A
d) 155.1 A
Answer: c
Clarification: The total emf can be calculated as: E = E₁ + E₂ + E₃ + …………… + E_N
(According to the relation given) = 1.5r₁ + 1.5r₂ + 1.5r₃ + ………… + 1.5r_N
= 1.5 (r₁ + r₂ + r₃ + ………… + r_N) → X
Total resistance can be calculated as: R = r₁ + r₂ + r₃ + ……….. + r_N → Y
Substituting Y in X
E = 1.5 × R
We also know that → I = (frac {E}{R})
So, I = (frac {E}{R}) = 1.5 A
Therefore, the current in the circuit = 1.5 ampere

10. Which of the following devices is the more accurate one for the measurement of emf?
a) Meter Bridge
b) Voltmeter
c) Multimeter
d) Potentiometer
Answer: d
Clarification: Potentiometer is the more accurate device to measure emf than the other ones such as multimeter or voltmeter. Potentiometer is highly sensitive and thus, even small emfs can be measured using this device. Moreover, potentiometers do not draw current from the circuit during measurements, like voltmeters.

Physics Exam Questions for Schools,

Physics Multiple Choice Questions on “Bar Magnet”.

1. Which among the following is not attracted by magnets?
a) Iron
b) Cobalt
c) Copper
d) Nickel
Answer: c
Clarification: A magnet is a material that has both attractive and directive properties. It attracts small pieces of iron, nickel, cobalt, etc. This property of attraction is called magnetism. Going by this criteria, copper is not attracted by magnets.

2. Identify the direction in which a thin long piece of magnet comes to rest when suspended freely.
a) East-west
b) North-south
c) Northeast-southeast
d) Northwest-southwest
Answer: b
Clarification: When suspended freely, a thin long piece of magnet comes to rest nearly in the geographical north-south direction. When placed in a non-uniform magnetic field, it tends to move from weaker to stronger magnetic field.

3. Which of the following is not a basic property of magnets.
a) Magnet exists as a monopole
b) Attractive property
c) Directive property
d) Like poles repel and unlike poles attract
Answer: a
Clarification: Attractive property, directive property and the law of magnetic poles are some of the important properties of magnets. So unlike electric charges, magnetic monopoles do not exist. Every magnet exists as a dipole.

4. Give the SI unit of magnetic dipole moment.
a) A^-2m
b) Am^-2
c) A²m
d) Am²
Answer: d
Clarification: The magnetic dipole moment of a magnetic dipole is defined as the product of its pole strength and magnetic length. The SI unit of magnetic dipole moment is ampere metre² (Am²). Magnetic dipole moment is a vector quantity and is directed from south to north pole of the magnet.

5. Which of the following is a natural magnet?
a) Magnetic needle
b) Bar magnet
c) Magnetite
d) Horseshoe magnet
Answer: c
Clarification: Pieces of naturally occurring iron ore, lodestone or magnetite had the property of attracting small pieces of iron. Hence, magnetite is a natural magnet. All natural magnets are permanent magnets, meaning they will never lose their magnetic power.

6. Magnetic lines of force always form open loops. State true or false.
a) True
b) False
Answer: b
Clarification: Magnetic poles always exist in pairs. So magnetic lines of force run from N-pole to S-pole outside the magnet and from S-pole to N-pole inside the magnet. They do not start or end anywhere and always forms closed loops.

7. A magnet can only repel another magnet. So, ‘X’ is a surer test of magnetism. Identify X.
a) Directive
b) Repulsion
c) Remanence
d) Hysteresis
Answer: b
Clarification: A magnet can attract another magnet or a magnetic substance like iron. However, a magnet can repel another magnet only. So repulsion is the surer test of magnetism.

8. A magnetic dipole of length 10 cm has pole strength of 20 Am. Find the magnetic moment of the dipole.
a) 2 Am²
b) 200 Am²
c) 20 Am²
d) 0.2 Am²
Answer: a
Clarification: Magnetic dipole moment = Pole strength × Magnetic length.
Magnetic dipole moment = 20 Am × 0.1 m
Magnetic dipole moment = 2 Am².

9. A magnetized needle of the magnetic moment 20 JT^-1 is placed at 60^o with the direction of the uniform magnetic field of magnitude 9 T. What is the torque acting on the needle?
a) 225 J
b) 625 J
c) 155.8 J
d) 318 J
Answer: c
Clarification: Torque, τ = mBsin (θ).
Given: m = 20 J/T; B =9 T; θ = 60^o
τ = 20 JT^-1 × 9 T × sin(60^o)
τ = 155.8J.

10. A bar magnet of the magnetic moment 5 Am² has poles 20 cm apart. Calculate the pole strength.
a) 250 Am
b) 4 Am
c) 100 Am
d) 25 Am
Answer: d
Clarification: Magnetic dipole moment = Pole strength × Magnetic length.
Pole strength = (frac {Magnetic , dipole , moment}{Magnetic , length})
Pole strength = (frac {5}{0.2})
Pole strength = 25 Am.

Physics Multiple Choice Questions on “AC Voltage Applied to an Inductor”.

1. What is the reactance of an inductor in a dc circuit?
a) Maximum
b) Minimum
c) Zero
d) Indefinite
Answer: c
Clarification: The reactance of an inductor in a dc circuit is zero.
For dc ➔ f = 0
X_L = 2π f L
X_L = 0
Therefore, the reactance of an inductor in a dc circuit is zero.

2. In which type of circuit the value of power factor will be minimum?
a) Resistive
b) Inductive
c) Superconductive
d) Semi conductive
Answer: b
Clarification: For a purely inductive circuit, Φ = ± (frac {pi }{2})
Power factor = cos (±(frac {pi }{2}))
Power factor = 0
Therefore, the power factor will be minimum for an inductive circuit.

3. The frequency of ac is doubled. How does X_L get affected?
a) X_L gets doubled
b) X_L becomes zero
c) X_L is halved
d) X_L is indefinite
Answer: a
Clarification: When the frequency of an ac is doubled ➔ The inductive reactance (X_L) gets doubled.
This is because inductive reactance is directly proportional to the frequency of an alternating current circuit.

4. At what frequency will a coil, which has an inductance of 2.5 H, have a reactance of 3500 Ω?
a) 700Hz
b) 350 Hz
c) 200 Hz
d) 223 Hz
Answer: d
Clarification: f = (frac {X_L}{2pi L})
f = (frac {3500}{(2 times 3.14 times 2.5)})
f = 222.9 Hz ≈ 223 Hz
Therefore, the frequency of a coil having reactance of 3500 Ω is 223 Hz.

5. Why does an inductor offer an easy path to dc and a resistive path to ac?
a) X_L is maximum for dc and infinite for ac
b) X_L is zero for dc and infinite for ac
c) X_L is zero for dc and finite for ac
d) X_L is maximum for dc and finite for ac
Answer: c
Clarification: For dc, f = 0,
X_L = 2π f L
X_L = 0
X_L is zero for dc and has a finite value for ac Hence an inductor offers an easy path to dc and a resistive path to ac.

6. The inductive reactance for an ac circuit containing only an inductor is (frac {E_0}{X_L}).
a) True
b) False
Answer: b
Clarification: No, this statement is false. The inductive reactance for an ac circuit containing only an inductor is X_L = ωL.
X_L = ωL
X_L = 2π f L

7. Determine the peak current if an inductor of inductance 500 mH is connected to an ac source of peak emf 650 V and frequency 100 Hz
a) 1.55 A
b) 2.07 A
c) 7.89 A
d) 9.87 A
Answer: b
Clarification: Peak current (I₀) = (frac {E_0}{X_L})
I₀ = (frac {650}{(2 times 3.14 times 100 times 0.5)})
I₀ = 2.07 A
Therefore, the peak current is calculated as 2.07 A.

8. Find out the rms value of current in the circuit wherein a 35 mH inductor is connected to 200 V, 70 Hz ac supply.
a) 13 A
b) 15 A
c) 20 A
d) 45 A
Answer: a
Clarification: X_L = 2π f L
X_L = 2π × 70 × 35 × 10^-3 Ω.
I_rms = (frac {E_{rms}}{X_L} = frac {200}{(2pi times 70 times 35 times 10^{-3})})
I_rms = 12.99 A ≈ 13 A
Therefore, the rms value of current in the circuit is 13 A.